Liquid crystals are suitable as dielectrics in indicating devices, since their optical properties can be influenced by an electrical potential. Suitable electro -optical devices are well-known to the person skilled in the art. Examples of such devices are cells having dynamic scattering, DAP cells (deformation of aligned phases), TN cells (twisted-nematic) and STN cells (super twisted-nematic) having a twisted nematic structure, guest/host cells, phase-change cells having a cholesteric-nematic phase transition and SBE cells (super birefringence effect).
Further, electro-optical devices based on chiral tilted smectic liquid crystals are proposed in Appl. Phys. Lett. 36, 899 (1980) and in Recent Developments in Condensed Matter Physics 4, 309 (1981). In this case the ferroelectric properties of these materials are made use of. As the tilted smectic phases there are suitable, for example, smectic C, F, G, H, I and K phases. There are generally preferred smectic C phases which permit especially high response speeds. The chiral tilted phases are usually denoted as S.sub.C *, S.sub.F * etc., with the asterisk indicating the chirality. Liquid crystals should have a good stability towards chemical and thermal influences and towards electric and magnetic fields. Further, they should have a suitable mesophase over a broad temperature range, low viscosity and short response times. Ferroelectric liquid crystals should preferably have a broad chiral smectic C phase and a sufficiently high spontaneous polarization. In order to facilitate the orientation in the cell, they can further preferably have a smectic A phase above the smectic C phase.
Liquid crystals are generally used as mixtures of several components in order to better optimize the various properties. Novel liquid crystal components should therefore preferably have a good miscibility with known materials.
As ferroelectric liquid crystal mixtures there are primarily suitable mixtures consisting of one or more optically active doping substances and a liquid crystal material (from one or more components) which as a rule should have a broad tilted smectic phase, preferably a smectic C phase. The optically active doping substances need not themselves be smectic, but preferably can have a smectic or cholesteric phase. However, in the liquid crystal material the optically active doping substances should produce a chiral tilted smectic phase and, further, should induce a sufficiently high spontaneous polarization and a comparatively small twisting. The pitch of the twisting should preferably be significantly larger than the plate separation of the cell which is used and typically should amount to at least about 10 .mu.m in order to obtain bistable displays having goo switching.